AP1 CH15: Special Senses

Gustation

- Taste

- Molecules have to be dissolved in water

- Flavor in food involves smell, texture, and appearance

- Cranial nerves VII, IX, X send taste sensations to the gustatory nucleus of the medulla oblongata

Tongue and taste bud diagram

Sweet, salty, bitter, sour, and savory (umami)

Ageusia

Absence of taste

Hypogeusia

Decrease in taste sensation, is not strong

Causes of ageusia and hypogeusia

- Upper respiratory tract infection

- Drugs: valium, amphetamines, tranquilizers, etc.

- Injury to unmyelinated nerves or to cranial nerves leading from oral mucosa to brainstem

- Oral infection/inflammation

- Mucositis damages microvilli

- Nutritional deficiencies: zinc, copper, nickel

- Inherited (Riley-Day Syndrome)

Treatments for ageusia and hypogeusia

-Treat the pathology/infection

- Address vitamin deficiency or hormonal imbalance

Olfaction

- Smell

Anatomy of olfaction

Olfactory mucosa on the roof of the nasal cavity contains 10-20 million olfactory neurons, each has 10-20 cilia (olfactory hairs)

Chemicals must be volatile and water-soluble

Physiology of Olfaction

Some odors stimulate nociceptors that trigger the trigeminal nerve

Quick to adapt

Olfaction mechanism

1. Odorant binds to receptor

2. Receptor activates G protein

3. G protein activated adrenylate cyclase

4. Adrenylate cyclase converts ATP to cAMP

5. cAMP opens a cation channel, allowing Na+ and Ca2+ influx and causing depolarization

Smell diagram

Anosmia

- Caused by head injuries; tear the olfactory nerves, nasal cavity inflammation, or aging

- Obstruction by polyps; treatment includes topical or systemic corticosteroid or surgical removal of polyps

- Damage to sensory epithelium after severe cold/allergy/smoking

- Zinc deficiency

Lateral view of eye

- conjunctiva: lines eyelids as palpebral, lines the eyeball as ocular; lubricates and protects

- lacrimal apparatus: lacrimal gland and associated ducts, secretes tears

Eyebrow

To reduce glare

Eyelids (palpebrae) and Conjunctiva

Protection

Lacrimal Apparatus

Tear ducts and the glands associated with making tears

To flush the eyes out, keep moist

Contain mucus/antibodies/lysozyme, enter the eye through superolateral excretory ducts, exit the eye medially through lacrimal punctum, and drain into nasolacrimal duct

Extrinsic eye muscles

- Two basic types of eye muscles

• Saccades movements

• Scanning movements: tracking moving objects

Maintains shape of eyeball

Saccades

Small, jerky movements

Superior rectus

Elevates eye and turns it medially

Inferior rectus

Depresses eye and turns it medially

Medial rectus

Moves eye medially

Lateral rectus

Moves eye laterally

Superior oblique

Depresses eye and turns it laterally

Inferior oblique

Elevates eye and turns it laterally

Conjunctivitis

Pink eye

Contagious, can be caused by infection

Treatment: eye drops, medicine

Strabismus

- Congenital weakness of the external eye muscles

- Affected eye rotates medially/laterally

- Treatments: eye exercises, surgery

Olfactory cilia

Receptors for smell

Olfactory sensory neuron

A bipolar (double-ended) nerve cell that captures odorant molecules and initiates the neural signals for smell.

Olfactory stem cell

Replace old sensory neurons every few months

Anatomy of the eye

- Three tunics (layers)

- Tunica fibrosa

- Tunica vasculosa

- Tunica interna

~ Ciliary body

~ Iris controls the diameter of the pupil, contains chromatophores with varying quantities of pigment

Tunica fibrosa

Outer layer that includes sclera and transparent cornea

Tunica vasculosa

Middle layer that has a layer to keep the inner eye dark, has a choroid, supplies eye tissue with oxygen and nutrients

Tunica internosa

Inner layer that is the retina

Ciliary body

Forms a muscular ring around the lens, changing the shape of the lens to focus

B

Suspensory ligament

Connects the lens to the ciliary body

Cornea

G, the transparent layer forming the front of the eye

The clear tissue that covers the front of the eye

Iris

E, a ring of muscle tissue that forms the colored portion of the eye around the pupil and controls the size of the pupil opening

- iris is two muscles; sphincter pupillae and dilator pupillae

Pupil

- Sphincter pupillae muscle contracts with parasympathetic activation (pupil constricts)

- Dilator pupillae muscle contracts with sympathetic activation (pupil dilates)

F

Lens

H, the transparent structure behind the pupil that changes shape to help focus images on the retina

Aqueous humor

Anterior segment of eye

G

Vitreous humor

Posterior segment of eye

J

Sclera

K, white of the eye

Retina

- Most posterior layer of the retina is pigmented epithelium

- Neural apparatus has three principal cell layers

- Contains sensory receptors that process visual information and sends it to the brain

retinal cells

- rods: contain rhodopsin (pigment) for colorless vision in dim light

- cones: bright light, sharp color vision

- bipolar cells (middle layer): synapse with photoreceptors and ganglion cells after receiving an image

- ganglion cells (outer layer): receive input from bipolar neurons

Macula lutea

N, a yellowish central area of the retina that is rich in cones and that mediates clear detailed vision

- Directly posterior to the lens

- Contains the fovea centralis

Fovea centralis

O, area consisting of a small depression in the retina containing cones and where vision is most acute

Optic nerve

The nerve that carries neural impulses from the eye to the brain

Optic disk

A hole in the retina where the optic nerve fibers exit the eye, no vision here (blind spot)

Q

Choroid

L; Middle, vascular layer of the eye, between the retina and the sclera

Central artery and vein of the retina

19

Supply retina with nutrients, only area in body where blood vessels can be directly observed

Ora serrata

A, the serrated boundary between the ciliary muscle and the retina

Suspensory ligament

C, attaches the lens to the ciliary body

Scleral venous sinus

Drains the aqueous humor from the eye

Optical apparatus

Admits and refracts light rays, then focuses them on the retina

Parasympathetic stimulation

Constricts the eye with the sphincter pupillae muscle

Sympathetic stimulation

Dilates the eye with the dilator pupillae

Neural apparatus

Has three principal layer

Rods

Contain the pigment rhodopsin

Colorless vision in dim light

Cones

Bright light and are responsible for both sharp and color vision

Bipolar cells

The middle layer, are those that rods and cones synapse with after receiving an image

Ganglion cells

Receive input from bipolar neurons

Glaucoma

- Drainage of aqueous humor is blocked, pressure increases, retina and optic nerve compressed, looks like tunnel vision

- Treatment: eye drops (miotics) increase rate of aqueous humor drainage, and possible surgery

Cataract

- "Waterfall"

- Lens thickens and hardens: inadequate nutrients to deep lens fibers, clumping of crystallin proteins

- Treatment: surgically remove defective lens, replace with artificial one

Emmetropic eye

Normal, focal point on retina

Myopic eye (myopia)

Eyeball too long, causes myopia (nearsightedness), focal point is in front of the retina; concave lens

Hyperopic eye (hyperopia)

Eyeball too short, causes hyperopia (farsightedness), focal point is behind the retina; convex lens

Nyctalopia

- Night blindness

- Rod function is defective

- Most common cause is prolonged vitamin A deficiency, rod degeneration

Image formation

Pupillary constrictor narrows the pupil to admit less light to the eye

Pupillary dilator widens the pupl to admit more light

Photopupillary reflex

Photo pupillary reflex

Reflex; the constriction of pupils when they are exposed to bright light, consensual

Refraction

- As light enters the eye, it's refracted by the cornea

- The aqueous humor doesn't reflect light well

- Most of the bending of light occurs in the lens

Near response

- Lens must change shape to focus light on the retina (flattens for distance, bulges for close) by means of ciliary muscle

- Convergence of the eyes orients visual axis to focus on the fovea centralis of each eye

- Lens accommodates; constriction of the pupil adjusts the amount of light, reduces spherical aberration

- Elasticity decreases with age

Near point of vision

The closest an object can be and still come into focus

Color vision

- Made possible by three sets of cones named for the absorption peaks of their visual pigments

- Blue: 420nm

- Green: 531nm

- Red: 558nm

Color blindness

- Lack of one or more cone types

- Most common in males (8-10%)

- Red-green is most common

Stereoscopic vision

Combination of two retinal images to give a 3-D perceptual experience; depth perception

Visual projection pathway

- The optic nerves converge to form optic chiasma. Then, fibers continue as optic tracts

- Hemidecussation

- Optic tracts pass around the hypothalamus to the lateral geniculate body of the thalamus

- Second-order neurons arise here to form optic radiation of fibers in the cerebrum

- These projects to the primary visual cortex of the occipital lobe (conscious perception of image)

Hemidecussation

Occurs within the chiasma, is an X

Sound

Any audible vibration of molecules

Measured in pitch and loudness

External ear

- 3 parts

- Outer auricle

- Auditory canal

- Tympanic membrane (eardrum) is very sensitive to pain

Pitch

- Determined by the frequency of vibration

- Human ears hear from 20-20,000 HZ (hertz)

Loudness

- Perception of amplitude (intensity) of frequency

- Loudness is expressed in decibels, with 120-140 causing pain in most people

Hearing loss

Prolonged exposure to sounds over 90 dB

Auricle

The outer projection, funnels vibrations toward the auditory canal and eardrum

External auditory canal

Leads to eardrum, lined with protective ceruminous glands and hairs

Middle ear

- Malleus (hammer)

- Incus (anvil)

- Stapes (stirrups)

Inner ear

- Housed within temporal bone

- Between bone and membrane is a fluid called perilymph

- Endolymph fills the chamber with the membranous labyrinth

Perilymph

Fluid between bone and membrane of inner ear

Endolymph

Fills chamber with the membranous lymph

Organ of Corti

- Basilar membrane supports the organ of Corti containing hair cells, each with stereocilia

- The tips of the stereocilia shear against an overlying tectorial membrane

- IHCs and OHCs

Inner hair cells (IHCs)

Contains stereocilia, send actual hearing impulses

Outer hair cells (OHCs)

Contains stereocilia, adjust the response of the cochlea to different frequencies

Physiology of hearing

- Auditory ossicles function: to concentrate the energy from the eardrum to a smaller oval window:

The ossicles and eardrum are protected by the tympanic reflex in response to loud noises (not effective for sudden loud noises)

The middle-ear muscles tighten up before you speak to protect your ears from the volume of your own voice

Stimulation of Cochlear Hair Cells

- Auditory ossicles vibrate against the oval window (sets up vibrations within the fluid-filled inner ear)

- Endolymph of cochlear duct vibrates, causing hair cell stereocilia to move against tectorial membrane

- Bending of stereocilia causes depolarization of hair cell; bending in the opposite direction closes the potassium ion channel while the cell hyperpolarizes

- Hair cell releases neurotransmitter during depolarization, generating an AP to cochlear nerve

Sound transduction

- Loud sounds produce vigorous vibrations of Organ of Corti, exciting more cells over larger area

- Higher AP frequency = loud sound

- A sound causes a standing wave in the basilar membrane

- Lower frequency sounds cause a peak amplitude at distal end of the organ of Corti

- Higher frequency sounds are detected closer to the proximal end

Conduction deafness

Hinderance to sound conduction (earwax buildup, perforated eardrum)

• Earaches, otitis media and otosclerosis of ossicles

• Surgical treatment, replace ossicles or excess tissue

Sensorineurial deafness

Direct damage to neural structurews

• Presbycusis, the inability to hear high-pitched sounds

• Common because of age and exposure to loud noises

• Cochlear implants

Equilibrium

Static and Dynamic Equilibrium

Maculae and utricle (horizontal) and saccule (vertical) respond to changes in gravity and linear acceleration

Static equilibrium

When you’re sitting or standing, still

Dynamic equilibrium

During movement

Meniere’s syndrome

Labyrinth disorder affecting both semicircular canals and the cochlea

• required attacks of vertigo (dizziness), nausea, vomiting, difficulty standing, tinnitus (ringing or clicking in ears)

• Treatment: mild = attention drugs, moderate = low-salt diet/diuretics to decrease endolymph volume, severe = surgically drain excess endolymph